This invention relates to evaporative emission control systems of the type that are commonly used on automotive vehicles for the purpose of minimizing the emission of hydrocarbon fuel vapors to the atmosphere.
A typical evaporative emission control system functions to collect hydrocarbon vapors in a canister and periodically purge the canister by venting the accumulated vapors to the intake manifold of the engine for subsequent combustion in the engine combustion chambers. Purging of the canister occurs periodically whenever conditions that are conducive to purging exist. Purging is conducted through a purge valve. A common type of purging control comprises the use of an electrically actuated valve to control the extent to which the vapors are allowed to pass to the intake manifold. The electrically actuated valve is under the control of a signal from the engine electronic control unit. The signal from the electronic control unit modulates a pulse width modulated signal to the solenoid valve such that the amount of purging that is permitted by the purge valve is controlled in accordance with certain engine operating conditions.
The present invention relates to a new and improved evaporative emission control system in which the purging process is better adapted to engine operating conditions. Rather than endeavoring to accomplish complete control of purging by means of a solenoid actuated valve, the present invention comprises a purge valve in which the regulating valve portion is preceded by a variable orifice valve portion. The variable orifice valve portion sets an orifice size that is in inverse relation to the magnitude of a manifold vacuum. The cooperative effect of the two valve portions of the purge valve is such that the flow through the purge valve as a function of the percentage duty cycle that is applied to the purge valve by the electronic vacuum regulator is defined by a series of graph plots each of which is defined by a particular magnitude of manifold vacuum. As a result, better control of canister purging is obtained and this has a special advantage during light engine load conditions (when intake manifold vacuum levels are high).
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.